What amp are you using? You're biggest problem is that the 3x10 has a wierd nominal impedance. From memory it's about 5 ohms. Most amps will allow a minimum of 4 ohms. So to add a cab will almost certainly bring your impedance below 4 ohms.

I've always wondered how Epifani users get around this. Use an amp that goes down to 2 ohms, or a power amp with 2 chanels I guess?

Once you've sorted the impedance issuse - Avatar seem to make gret value for money cabs. Have a look at their 15's.

Good point. The T-310 is rated at 5.6 ohms. Adding an eight ohm cabinet would take it a little below 4 ohms. I'm using a Mesa Walkabout which prefers 4 ohms but can go down to 2, but it tends to overheat when pushed.

I've been looking at the Avatars, also used Bag End 15's. I hear good things about both.

If it says it can go down to 2 ohms than it probably can. Don't worry about heat unless it's really really hot. Amps will always get hot. But if you're worried, put a small fan on it. The one I use is 12V, 8cm by 8cm (ie tiny. Same height as a 2 ru amp) and does a tremendous job of keeping things cool.

i always treat 3 driver cabs as if they're 4 ohms, dont get to hung up on impedance it is nominal

overall, if your amp is rated for 2 oms and is mono, i would get another 4 ohm cab, using a 4 and an 8 on the same load really doesnt do well, as the 8 wont get pushed enought to get heard
if youre using a stereo amp then you can use whatever you want on the other side

for 8 ohm cabs the epi 112 is definitely worth a listen, and i would strongly reccomend a bagend 15 with or without the coaxial tweeter, they are incredible

for my $. i would probably use a stereo amp and get an epii 212 for the other side